either setting up a discount to that effect with this
https://support.wp-pizza.com/downloads/wppizza-coupons-and-discounts/ (i.e if 2 pizzas usually cost 15 each, your discount for 2 would be 10. as in 15+15-10 = 20)

or you could also use the add ingredients extension somewhat (see examples here https://demo.wp-pizza.com/wppizza-add-ingredients/our-menu/user/ – perhaps the “SIMPLE MEAL FOR 2” example)
however, add ingredients does not then allow you to add more ingredients to a subitem , so you could not then choose what to have on each pizza , as that’s not really what it’s designed for

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please note:
The “Add Ingredients” extension is NOT a “Special Offer” or “Meal Deal” or similar type plugin and was never conceived for this .
(although – clearly- many things are possible as you can see in the user examples https://demo.wp-pizza.com/wppizza-add-ingredients/our-menu/user/ that are not really within the scope of the plugin)

At some – undetermined – point in the future I *might* create a dedicated plugin for this sort of thing or will actually add a “add product-to-product” (or whatever it’s going to be called ) to the ” Add Ingredients” plugin
That will allow *menu items* to be added to other menu item, but at the moment this idea is only in the “ideas” / “perhaps an idea to do one day” stage